Simulation for stiffening of reconstituted silk fibroin gels during deformation

Silk from silkworms has been used in the textile industry for thousands of years. It consists of β-sheet structures due to hydrophobic domains. Large hydrophobic domains interspaced with smaller hydrophilic domains generate the strength and impressive mechanical property of silk. Recently, a physical electrogel(e-gel) was made by reconstituting Bombyx mori silk into stable aqueous solutions and then applying small DC electric field [Tabatabai et al, Soft Matter 11 (2015) 756]. The e-gels exhibit distinctive strain hardening and are partially recoverable from strain. To study the structure change and nonlinear behavior of the gel, we build a coarse-grained model of fibroin protein polymers, which comprise crystallizable domains and amorphous domains. We find that the kinetics of unfolding and refolding of crystallizable domains changes the number and functionality of crosslinks in the physical network, and thus contributes to the strain hardening of the gel and the non-recoverable strain.